Microscopic Structure of Bone

Microscopic Structure of Bone
Compact bone is composed of a calcified
bone matrix arranged in concentric
rings. The rings contain cavities
(lacunae) filled with bone cells
(osteocytes), which are interconnected
by many minute passages (canaliculi). These passages serve to
distribute nutrients throughout the
bone. This entire organization of lacunae
and canaliculi is arranged into an
elongated cylinder called an osteon
(also called haversian system) (Figure
31-7). Bone consists of bundles of
osteons cemented together and interconnected
with blood vessels and
nerves. Because of blood vessels and
nerves throughout bone, it is living tissue,
although nonliving “ground substance”
predominates. As a result of its
living state, bone breaks can heal, and
bone diseases can be as painful as any
other tissue disease.

Following menopause, a woman loses 5% to
6% of her bone mass annually, often leading
to the disease osteoporosis and increasing
the risk of bone fractures. Dietary supplementation
with calcium has been advocated
to prevent such losses, but even large
doses of calcium alone have little effect in
slowing demineralization unless accompanied
by therapy with the female sex hormone
estrogen (because ovarian production
of estrogen drops significantly after
menopause). Among animals, only humans,
especially females, are troubled with osteoporosis,
perhaps a consequence of the long
postreproductive life of the human species.

Bone growth is a complex
restructuring process, involving both
its destruction internally by boneresorbing
cells (osteoclasts) and
its deposition externally by bonebuilding
cells (osteoblasts). Both
processes occur simultaneously so
that the marrow cavity inside grows
larger by bone resorption while new
bone is laid down outside by bone
deposition. Bone growth responds to
several hormones, in particular parathyroid hormone from the
parathyroid gland, which stimulates
bone resorption, and calcitonin from
the thyroid gland, which inhibits
bone resorption. These two hormones,
together with a derivative of
vitamin D, are responsible for maintaining
a constant level of calcium in
the blood. The effect of hormones on
bone growth and resorption is
described in more detail on.

BoneLike muscle, bone is subject to “use and disuse.”
When we exercise our muscles, our
bones respond by producing new bone tissue
to give added strength. In fact, thebumps and processes to which muscles
attach are produced by bone in response to
the action of muscle forces. Conversely,
when bones are not subject to stress, as in
space flight, the body resorbs the mineral,
and the bones become weak. Astronauts
who spend many months in space must be
carried from their capsules upon their
return to earth.

Plan of the Vertebrate Skeleton
The vertebrate skeleton is composed
of two main divisions: axial skeleton, which includes skull, vertebral column,
sternum, and ribs, and appendicular skeleton, which includes the limbs (or
fins or wings) and pectoral and pelvic
girdles (Figures 31-8 and 31-9). Not
surprisingly, the skeleton has undergone
extensive remodeling in the
course of vertebrate evolution. The
move from water to land forced dramatic
changes in body form. With
increased cephalization, the further
concentration of brain, sense organs,
and food-gathering and respiratory
apparatus in the head, the skull
became the most intricate portion of
the skeleton. Some early fishes had as
many as 180 skull bones (a source of
frustration to paleontologists) but
through loss of some bones and fusion
of others, skull bones became greatly
reduced in number during evolution of
the tetrapods. Amphibians and lizards
have 50 to 95, and mammals, 35 or
fewer. Humans have 29.

The vertebral column is the main
stiffening axis of the postcranial
skeleton. In fishes it serves much the
same function as the notochord from
which it is derived; that is, it provides
points for muscle attachment and prevents
telescoping of the body during
muscle contraction. With evolution of
amphibious and terrestrial tetrapods,
the vertebrate body was no longer
buoyed by the aquatic environment.
The vertebral column became structurally
adapted to withstand new
regional stresses transmitted to the column
by the two pairs of appendages.
In amniote tetrapods (reptiles, birds,
and mammals), the vertebrae are differentiated
into cervical (neck), thoracic
(chest), lumbar (back), sacral
(pelvic), and caudal (tail) vertebrae. In
birds and also in humans the caudal
vertebrae are reduced in number and
size, and the sacral vertebrae are fused.
The number of vertebrae varies among
the different vertebrates. Pythons
seems to lead the list with more than
400. In humans (Figure 31-9) there are
33 in a young child, but in adults 5 are
fused to form the sacrum and 4 to
form the coccyx. Besides the sacrum
and coccyx, humans have 7 cervical,
12 thoracic, and 5 lumbar vertebrae.
The number of cervical vertebrae (7)
is constant in nearly all mammals, whether the neck is short as in dolphins,
or long as in giraffes.

5

Figure 31-9
Human skeleton. A, Ventral view. B, Dorsal view. In comparison with other mammals, the human skeleton is a patchwork of primitive and specialized parts.
Erect posture, brought about by specialized changes in legs and pelvis, enabled the primitive arrangement of arms and hands (arboreal adaptation of
human ancestors) to be used for manipulation of tools. Development of the skull and brain followed as a consequence of the premium natural selection put
on dexterity and ability to appraise the environment.

The first two cervical vertebrae, atlas and axis, are modified to support
the skull and permit pivotal movements.
The atlas bears the globe of the
head much as the mythological Atlas
bore the earth on his shoulders. The
axis, the second vertebra, permits the
head to turn from side to side.

Ribs are long or short skeletal
structures that articulate medially with
vertebrae and extend into the body
wall. Fishes have a pair of ribs for
every vertebra (Figure 31-8); they serve
as stiffening elements in the connective
tissue septa that separate the muscle
segments and thus improve the effectiveness
of muscle contractions. Many
fishes have both dorsal and ventral
ribs, and some have numerous riblike
intermuscular bones as well—all of
which increase the difficulty and
reduce the pleasure of eating certain
kinds of fish. Other vertebrates have a
reduced number of ribs, and some,
such as the familiar leopard frog, have
no ribs at all. In mammals the ribs
together form the thoracic basket,
which supports the chest wall and
prevents collapse of the lungs. Mammals
such as sloths have 24 pairs of
ribs, whereas horses posses 18 pairs.
Primates other than humans have 13
pairs of ribs; humans have 12 pairs,
although approximately 1 person in 20
has a thirteenth pair.

Figure 31-8
Skeleton of a perch

Most vertebrates, fishes included,
have paired appendages. All fishes
except agnathans have thin pectoral
and pelvic fins that are supported by
the pectoral and pelvic girdles, respectively
(Figure 31-8). Tetrapods (except
caecilians, snakes, and limbless lizards)
have two pairs of pentadactyl (fivetoed)
limbs, also supported by girdles.
The pentadactyl limb is similar in all
tetrapods, alive and extinct; even when
highly modified for various modes of
life, the elements are rather easily
homologized (the evolution of the
pentadactyl limb is illustrated in Figure
27-1,).

Modifications of the basic pentadactyl
limb for life in different environments
involve distal elements much
more frequently than proximal, and it
is far more common for bones to be
lost or fused than for new ones to be
added. Horses and their relatives
evolved a foot structure for fleetness
by elongation of the third toe. In effect,
a horse stands on its third fingernail
(hoof), much like a ballet dancer
standing on the tips of the toes. The
bird wing is a good example of distal
modification. The bird embryo bears
13 distinct wrist and hand bones
(carpals and metacarpals), which are
reduced to three digits in the adult.
Most finger bones (phalanges) are lost,
leaving four bones in three digits (see
). The proximal bones (humerus,
radius, and ulna), however, are only
slightly modified in the bird wing.

In nearly all tetrapods the pelvic
girdle is firmly attached to the axial
skeleton, since the greatest locomotory
forces transmitted to the body come
from the hindlimbs. The pectoral girdle,
however, is much more loosely
attached to the axial skeleton, providing
the forelimbs with greater freedom
for manipulative movements.

Effect of Body Size
on Bone Stress
As Galileo realized in 1638, the ability
of animals’ limbs to support a load
decreases as animals increase in size
(section opening essay,). Imagine
two animals, one twice as long as
the other, that are proportionally identical.
That is, the larger animal is twice
as long, twice as wide, and twice as tall
as the smaller. The volume (and the
weight) of the larger animal will be
eight times the volume of the smaller
(2 × 2 × 2 × 8). However, the
strength of the larger animal’s legs will
be only four times the strength of the smaller, because bone, tendon, and
muscle strength are proportional to
cross-sectional area. So, as Galileo
noted, eight times the weight would
have to be carried by only four times
the strength. Because the maximum
strength of mammalian bone is rather
uniform per unit of cross-sectional
area, how can animals become larger
without placing unbearable stresses on
long limb bones? One obvious solution
is to make bones stouter and therefore
stronger. However, throughout much
of their size range, bone shape in different
sized mammals does not change
much. Instead, mammals have adapted
limb posture so that stresses are shifted
to align with the long axis of the
bones, rather than transversely. Small
animals the size of a chipmunk run in
a crouched limb posture, whereas a
large mammal such as a horse, has
adopted an upright posture (Figure
31-10). Bones and muscles are
capable of carrying far more weight
when aligned more closely with the
ground reaction force, as they are in a
horse’s leg. In this way, peak bone
stresses during strenuous activity are
no greater for a galloping horse than
for a running chipmunk or dog.

Figure 31-10 Comparison of postures in small and large mammals, showing the effect of scale. Because of its more upright posture, bone stresses in the horse are
similar to those in the chipmunk. In mammals larger than horses (above about 300 kg), greatly increased stresses require that bones become exceedingly
robust and that the animal lose agility.

For animals larger than horses, further
mechanical advantage by changing
limb posture is not possible
because the limbs are fully upright.
Instead, the long bones of an elephant
weighing 2.5 metric tons, and those of
the enormous dinosaur Apatosaurus,
weighing an estimated 34 metric tons,
are (were) extremely thick and robust
(Figure 31-10), providing the safety
factor these massive animals require(
d). However, top running speeds
of the largest terrestrial mammals
decline with increasing size. Nevertheless,
recent calculations of bone
stresses in dinosaurs suggest that even
the largest were capable of considerable
agility (Alexander, 1991).